Reticle Perpendicularity By Darrell Holland Long range shooters go through great pains to properly set-up their rifles for long range shooting. Bipods, muzzle brakes, cosine indicators and a host of other widgets often adorn their newly created...
Probably the best you can do is Brian Litz's tall target test. Youtube link below.
If that's not feasible, a box test is a good one. for a box test fire two shots at a point on the lower left of the target. Dial up 10 MOA and fire 2 or 3 shots at the same spot on the lower left. Impact points should be 10.47" higher and follow a perfectly vertical line above the original impacts. Dial 10 MOA right, repeat the shots while aiming at the original location. Dial 10 MOA down, repeat shots. dial 10 MOA left repeat the shots. all distances should be about 10.47"
below is a pick of a box test I did at 100 yards a few weeks ago.
I made a crude fixture to mount a scope and have concrete benches at the range I use so that I can dial a scope over a scaled piece of paper to see what the tracking accuracy is like. I've also done a Tall Target test as described by Bryan Litz.
One thing I will strongly caution you is to KNOW what the actual distance is from scope to paper/target